HEPA Air Purifiers
High Efficiency Particulate Arresting
(HEPA) filters have been around for quite some time. In the 1950s
the US Atomic Energy
Commission needed something to remove small particulates that are radioactive. Since then
HEPA air purifiers have been using this technology to clean the indoors
in cleanrooms to beauty parlors. HEPA products are also one of the many
techniques and products used by Allergy and Asthma doctors to help alleviate the
symptoms of those conditions. First we will discuss what HEPA really is.
Imagine a filter that only allows very tiny particles to
pass through. If we placed a single sheet of this HEPA paper in front of
a fan constrained as it would be in air purifiers, very little air would
be able to pass because of the small size of the holes. In addition, air
purifiers made like this would need to have the filter changed often
because the holes would plug rapidly. Now if we double the size of the
sheet we would get twice the airflow and filter life. You can not keep
growing the sheet larger because it would be impractical. Instead
let us try folding the paper back and forth so that we can present a
very large surface area to the airflow and thus the HEPA made
this way would be efficient. This is how real-world HEPA filters are
made in real HEPA systems. Some have as much as 40 square feet of the
filter material folded into the HEPA section. As a side note the material
from which HEPA is constructed is either fiber or paper-like on one hand
or a polymer on the other.
HEPA material does not look like a screen or a colander. Instead it
looks like a very thin bail of fibers. Thus the air has to find a
route through this maze of fibers. There are three ways the HEPA
filter stops particulates. First and the easiest to understand is
that a particle runs into a fiber and sticks. Secondly, the particle gets within one diameter
of a fiber of the HEPA filter and while it tries to "skid by" the fiber
it is gets stuck on the fiber. Third, as a very
small, about 0.1 micron, particle moves in the gas flow it dithers about
due to collisions with molecules (Brownian motion) and again
happen to slide close to a fiber and get caught.
Therefore, HEPA air purifiers
stop mold spores as well as many bacteria and viruses and of course
larger items such as dust. Most air
purifiers claim to be 99.97% efficient at removing particles 0.3 microns
from the air that passes through the HEPA filter. The
operative phrase is "pass through"! If the airflow does not have
an opportunity to pass through the HEPA filter it will not be cleaned.
Therefore the claim of 99.97% of all .3 and larger particles being
removed is not accurate in poorly designed HEPA air purifiers where some
of the airflow may pass around the HEPA filter and return to the room
not cleaned. The Austin Air HealthMate Plus series is the only line that specifies that
over 95% of the airflow that enters the air purifier goes through the
As good as that sounds pure HEPA air purifiers do not remove odors,
chemicals or gasses. Since these are molecular level substances the 0.3
micron holes are large compared to the gas molecules. Therefore typical
HEPA air purifiers have some level of activated carbon based material to
absorb odors and chemicals. The activated carbon that is included with all of
these units comes in a number of varieties. This varies from a
thin mat in an NeoAir unit, to pounds of activated carbon in an
AllerAir Air Purifier.
In any form the carbon absorbs gasses that will not be caught by the
HEPA element. These chemicals are either harmful gasses or those
that cause odors.
HEPA air purifiers are straight forward,
a fan forces airflow through a filter. The more times the airflow
goes through the filter in an hour the cleaner the room. So, the bigger
the room, the bigger the product, or our preference, the more small air
purifiers. Why? Think about a long narrow room. If you
put the product at one end how often is the air from the other end going
to get through the HEPA filter? Not often. If you put smaller
air purifiers at either end of the room the air will only have to move
half as far to get through the HEPA and get clean.
Activated Carbon in Home and Room Air
The presence of activated carbon in room air purifiers may be a mystery
to you, unless you're a aquarium hobbyist, or own a water purification system
which uses carbon. For many people, the first time they encounter it is in
selecting home air
purifiers. Although most of our customers begin their search because of allergies
or asthma, many soon realize how much odors and the chemicals with can cause
them also contribute to their respiratory problems. That's why most quality room air
purifier systems also address these issues.
II. History and Use in Purification Processes
As many as 2000 years ago, human beings were using activated carbon to
remove impurities from water. Even then it's exceptional adsorption qualities
were known. However, it was not until the early 1900's that it was produced in a
form (as powder) that could be sold commercially. At that time it was used to
purify water (so that it had no smell or taster) and to take the color out of
sugar. As World War I got into swing it was discovered that it could be used in
gas masks to protect soldiers, as well as for war time water and air
purifiers. Instead of being used in powder form, granular activated carbon
was developed. From that point on, activated carbon was used in many ways.
III. Why Is It Called
If you are unfamiliar with activated carbon and wonder why it is called
"activated" and if there is such a thing as non-activated carbon, this will
solve the mystery. The heat used in making carbon "activated" drives out
impurities so there are places for impurities from your air to reside when it is
used in an air purifier. After activated carbon is used for a while, it can
actually be reactivated by cooking it again. However, this is impractical and
dangerous. "Non-activated" carbon is soot or charcoal.
Activated carbon is described in a variety of ways, and there are a variety of
types, but generally it includes a wide range of amorphous carbon-based
materials which exhibit a high degree of porosity and an extended surface area.
In simple terms, this means that it has excellent absorbent characteristics that
make it very useful for a wide variety of filtration processes including air and
IV. How Is Activated Carbon
Used in Home Air Purifiers?
In room air purifiers, the activated carbon is often combined with other
minerals like zeolite. Zeolite can absorb ions and molecules and thus act as a
filter for odor control, toxin removal and as a chemical sieve. In some units,
the carbon may be impregnated with a potassium iodide or blended with
impregnated active alumina to increase the absorbent qualities. These home air
purifiers are particularly helpful to people with Multiple Chemical Sensitivity
(MCS), because they absorb formaldehyde which is found in carpet, wood paneling,
and furniture upholstery. Perfumes as well as chemicals in household cleaning
items are also removed, making the environment much more breathable for people
in general, but especially asthma sufferers, babies, children, and the elderly.
V. The Carbon in Room
The type and amount of activated carbon, other materials with which it
is blended, and how it is used in home air purifiers depends on the brand and model.
Here are the highlights:
Austin Air HM400 – contains 18 pounds of granular activated carbon impregnated
with zeolite to enhance chemical adsorption.
Austin Air HM400 SuperBlend (also called Plus) – contains 18 pounds of granular
activated carbon impregnated with a blend of zeolite and potassium to make it
effective at removing formaldehyde.
VI. Summary - Home Air
For many people, particulates such as pollens or pet dander are the main
irritants, with odor and chemical adsorption being less important or not
important at all. For others who need a high degree of both, it is best to look
at the higher caliber room air purifiers which offer outstanding particle, odor and
chemical removal. VOC's (volatile organic compounds), smog, ozone, fumes from
cooking, pets, or tobacco can be unhealthy or irritating for anyone not just
allergenics and asthmatics. If you're going to have home air purifiers anyway, it
only makes sense to get some that can absorb practically everything.